The present invention relates generally to motorized wheelchairs, and particularly concerns a wheelchair stability control system that advantageously and significantly reduces wheelchair tendencies toward veering and/or tipping during wheelchair operation.
Motorized wheelchairs with differential steering tend to be difficult to control. The wheelchair short wheelbase and narrow track necessary for maneuverability in close quarters makes it difficult to achieve wheelchair stability during operation at relatively high forward velocities. The stability problem is exacerbated in the typical front-wheel drive wheelchair by the use of trailing, freely-swiveling castors. In such wheelchairs the center of gravity is behind the wheelchair drive wheels and thus the momentum of the moving wheelchair adds to any turning force generated by the drive wheels. Once the front wheels lose traction, the trailing castors allow the wheelchair to spin out of control. In a rear-wheel drive wheelchair, where the center of gravity is ahead of the drive wheels the inertia of the wheelchair opposes turning forces generated by the drive wheels.
Also, in front-wheel drive wheelchairs the center of gravity with the occupant included is generally positioned at a level that is above the level of the drive wheel axis of rotation, and thus braking forces generated at the drive wheels during wheelchair forward motion, when combined with the momentum forces acting through the center of gravity, create wheelchair forward tipping moments which further contribute to wheelchair motion instability.
A typical maximum stable speed for rear-wheel drive wheelchairs is about 7+ miles per hour. The typical maximum speed for front-wheel drive wheelchairs is approximately 4 miles per hour if the drive wheel differential speed is monitored with motor current/voltage sensing. If the front drive wheels are monitored with tachometers and/or position sensors, speeds of approximately 5 miles per hour can be achieved. However, in either instance once the wheels start to slip, directional control is lost.
It is possible to further increase the maximum speed to about 6 or 7 miles per hour in a front-wheel drive wheelchair by steering the rear castors, but this approach sacrifices the maneuverability (turning radius) that predicted the selection of front-wheel drive to start. Such wheelchairs are not practical for indoor use.
An additional stability problem with both front-wheel drive and rear-wheel drive power wheelchairs is xe2x80x9cveeringxe2x80x9d when traversing a sloping surface. In those instances there often is a tendency for the wheelchair to uncontrollably turn or xe2x80x9cveerxe2x80x9d. Such can be an instability problem of major magnitude in wheelchairs that use switch-type operator controls.
Additional stability problems can occur with the failure of a rate-of-turn sensor.
The motorized wheelchair of the present invention includes a wheelchair chassis, a motorized right front-wheel connected to the chassis, a motorized left front-wheel connected to the chassis, and a trailing castor assembly also connected to the chassis. The invention wheelchair further includes an operator input device such as a conventional joystick control that generates, in response to operator manipulation, wheelchair forward/reverse linear velocity commands and wheelchair turn direction/turn rate commands. Also included in the invention wheelchair is a closed-loop servo control system controller subassembly that, in addition to receiving operator input turn and velocity commands, receives operating power from a battery source, and generates and utilizes control system feed-back inputs from included rate-of-turn, forward acceleration, lateral acceleration, and vertical acceleration sensors.
From an operating method standpoint, the wheelchair invention involves the steps of rotating two front wheel drive wheels at equal rates to cause wheelchair forward and/or reverse linear motion in accordance with corresponding input command signals, or differentially in response to wheelchair turn commands, of sensing the wheelchair turn direction and wheelchair turn rate with an angular rate-of-turn sensor, of comparing or summing the sensed wheelchair turn direction and wheelchair turn rate with the system input command turn direction and turn rate, and of differentially altering the motorized wheelchair wheel rotational speeds in response to the comparison step thereby obtaining improved wheelchair motion stability, especially with respect to reduction of wheelchair spin-out or veering tendencies and with respect to reduction of wheelchair tipping tendencies.
In addition, stability of the wheelchair can be increased by the inclusion of a detector for detecting the operation and/or the failure of the angular rate-of-turn sensor. Upon detection of the failure of the angular rate-of-turn sensor, the forward velocity of the wheelchair is limited to a maximum forward velocity that the wheelchair can obtain without the use of the rate-of-turn sensor. One advantage of limiting the wheelchair to a maximum forward velocity, rather than forcing the wheelchair to come to a complete stop is to allow the wheelchair occupant the limited mobility to continue moving without being completely stranded. Detecting the failure of the rate-of-turn sensor and the subsequent automatic control of the forward velocity has the advantage of preventing the wheelchair from going into an uncontrolled spin.
Failure of the rate-of-turn sensor may be detected by monitoring or sensing the signal of the rate-of-turn sensor. This can be accomplished by utilizing a second rate-of-turn sensor having an output which is equal in magnitude but opposite in polarity to the first rate-of-turn sensor for a given rate-of-turn. The outputs of each can be compared to one another, wherein the failure of a rate-of-turn sensor can be determined by a change in this comparison.